Aim: to compare the shaping ability of two rotary systems (Protaper Universal and Protaper Next) and K-file hand instruments in curved root canals of extracted human molars. Materials and methods:Thirty extracted molars with curved canals were chosen. The curvature of root canals was determined (15-40o) then the sample was randomly divided into three groups: n1=n2=n3=10. Group(1): was prepared using K-file hand instruments (Stand), group(2): prepared by Protaper Universal (PTU), whereas group3: prepared by Protaper Next (PTN).The curvature after preparation and Time preparationwere evaluated. Data were collected and statistical analysis was conducted. Results: The three instrument systems (PTU, PTN, Stand) have caused significant change in original canal curvature(P<0.05). On the other hand, there were no significant differences between systemsin straightening of canal curvature (P>0.05). However, both systems: PTN, Stand were significantly faster than PTU during preparation of curved canals (P<0,001).Conclusion: under the conditions of this study, it can be concluded thatall instruments caused changing in original canal curvature. However, the new generation PTN was faster than old one PTU.